1. Field of the Invention
The present invention relates to a pneumatic tire having blocks on the tread surface and a manufacturing process thereof. Specifically, it relates to a pneumatic tire which is excellent in running performance on an iced road.
2. Description of the Prior Art
In general, a car tire having sipes 14Z which communicate with the vertical groove 11 sides of blocks 13Z at both ends as shown in FIG. 9 and are formed on the surface of a block pattern formed on the tire tread surface to improve the gripping performance of a tire when a vehicle runs on an iced road is widely used. The above sipes 14Z increase the ground contact pressure of the tire due to an edge effect like the edges of the vertical grooves 11 or horizontal grooves 12, and each of the blocks 13Z is divided into a plurality of small blocks 13k by the above sipes 14z on the tire tread side, whereby the block 13Z is easily deformed. The above sipes 14Z cut a water film and form water channels to facilitate drainage when the water film is thick. Therefore, friction force between the road and the tire becomes large, the gripping performance of the tire 10 improves, and the running performance on an iced road of a vehicle enhances.
As for the shape of the above sipe 14Z, a flat (or wavy) sipe whose shape does not change in the depth direction, called “2D sipe” as shown in FIG. 10 is common in the prior art. However, when the sipe 14Z is the above 2D sipe and the number of sipes is increased to raise the friction coefficient, the stiffness of the whole block 13Z lowers and the above sipe 14Z falls, thereby reducing drainage efficiency and gripping performance.
To cope with this, a method of forming a so-called 3D sipe whose shape changes in the depth direction, such as sipes 14A to 14G shown in FIGS. 11(a) to 11(c), FIGS. 12(a) and 12(b) and FIGS. 13(a) and 13(b) in the block 13Z has recently been proposed. That is, by forming 3D sipes having a linear or wavy surface shape and inclined surfaces in the depth direction (radial direction of the block) of the block 13Z in the block 13Z, when longitudinal force such as braking or traction force is applied to the tire 10, the walls of the inclined surfaces of the 3D sipes come into close contact with each other to prevent the fall of the sipes, thereby making it possible to greatly improve the stiffness of the block.
Describing the above sipes 14A to 14G in detail, the sipe 14A shown in FIG. 11(a) is a 3D sipe having one pair of inclined surfaces in the depth direction of the sipe, the sipe 14B shown in FIG. 11(b) is a 3D sipe having two pairs of inclined surfaces in the depth direction of the sipe, and the sipe 14C shown in FIG. 11(c) is a 3D sipe having a series of projections with an equilaterally triangular section in the depth direction of the sipe.
The sipe 14D shown in FIG. 12(a) is a 3D sipe having a series of projections with a triangular section and a series of recesses with a triangular section in the axial direction of the tire both of which are arranged at a half pitch in the depth direction of the sipe, and the sipe 14E shown in FIG. 12(b) is a 3D sipe whose adjacent uneven walls in the depth direction are inclined inversely in the circumferential direction of the tire. The sipe 14F shown in FIG. 13(a) is a 3D sipe having alternating pyramidal projections and recesses in the tire axial direction and depth direction of the sipe, and the sipe 14G shown in FIG. 13(b) consists of a wavy 2D sipe on the front surface side and a 3D sipe having alternating pyramidal projections and recesses at a position deeper than the 2D sipe (refer to patent documents 1 to 4, for example).    Patent document 1: JP-A 11-310012.    Patent document 2: JP-A 2002-321509    Patent document 3: JP-A 2002-187412    Patent document 4: JP-A 2002-356105